FIG. 1 illustrates a known form of magnetic inertial force generator 60, including a cylindrical housing 62 defining internally an armature chamber 64 containing an armature 66 reciprocable along a central axis 68. The housing 62 is formed with a cylindrical magnetic steel shell 70 closed by aluminum end caps 71, 72 and containing a pair of axially spaced electric coils 73, 74 mounted on the interior of the housing 62.
The armature 66 includes a permanent magnet 76 having axially spaced north N and south S poles at opposite ends on which a pair of magnetic steel end plates 78 are mounted. The end plates 78 extend laterally to outer ends 80, forming a periphery of the armature 66 in general alignment with the electric coils 73, 74. The permanent magnet 76 generates a magnetic flux field concentrated in the steel end plates 78 and extending radially through the coils 73, 74 into the steel shell 70. Upper and lower resilient springs 82,84 between the end plates 78 and the end caps 71, 72 of the housing 62 are configured for nominally centering the armature 66 between the end caps within the chamber 64.
In operation, an AC voltage of controllable frequency is applied to the electric coils 73, 74 so that an AC current flows in the coils. The interaction of the magnetic flux from the armature 66 and the current in the coils creates a force on the armature and an equal and opposite force on the coil. Because the coils 73, 74 are rigidly attached to the outer shell 70, no force is transmitted outside the inertial force generator from the coil. As the armature 66 moves upward, the upper spring 82 is compressed and the lower spring 84 is extended. If the armature moves downward, the reverse is true. As the springs 82, 84 are compressed and expanded, the force is transmitted to the end caps 71 and 72. Because the end caps are rigidly attached to the outer shell 70, no force is transmitted outside the inertial force generator from the springs.
As the armature 66 moves up and down, the armature undergoes a time-varying position, velocity, and acceleration. Because the moving armature has mass, a force is required to maintain the position of the inertial force generator 60. Without equal and opposite force, the shell 70, coils 73 and 74, and end caps 82 and 84 would move downward together as the armature 66 moved upward to keep the center of mass of the inertial force generator in the same position. As a result, the inertial force generator 60 is capable of exerting force on a connected body for any desired purpose. Such purposes may include for example, vibration testing of manufactured assemblies, and reduction or cancellation of vibrations by application of forces opposite to the forces stimulating the motion of the vibration.